1. Field of the Invention
The present invention relates to methods and apparatus for monitoring and controlling the flow of flowable materials such as sand, gravel, or liquids. In particular the present invention relates to a method for monitoring and calibrating continuous, semi-continuous or intermittent feeding systems for discharging granular or powdered free flowing materials at known mass feed rates.
2. Related Art
As used herein, a flowable material is a material that can flow under the influence of gravity, including liquids such as water or syrup, and solids; substantially dry particulate materials such as sand, gravel, or alumina.
The aluminium industry has served as a stimulus for this work. The aluminium industry has matured in the period since the electrolytic reduction process was developed by Hall and Héroult in 1888. The method by which the alumina is fed to the melting cells is one particular area in which the industry has changed significantly. Changes in alumina feeding technology have been accelerated since 1960 when the size of cells increased above 100,000 amperes and environmental requirements associated with the growth of the industry meant that the cells have to be enclosed as much as possible. Mechanical automated systems have given way to point feeders where small amounts (in the range 0.5 to 3 kg typically) of alumina are added through the crust of the cell utilising a hole that has been pierced by a crust-breaker. This has moved smelting from a semi-batch operation to a more continuous one.
Recent trends in the smelting industry for improving the energy efficiency and performance of the cells have resulted in the electrolyte being modified to compositions that result in lower solubility ranges for the added aluminium oxide. Therefore, even with the smaller additions possible with point feeders, the process has a tendency to periodically or continuously form an aggregate or slurry of undissolved alumina and electrolyte that sinks to an inaccessible region below the liquid metal. This causes operating disturbances. A deficiency develops in the amount of alumina available, based on the assumed feed versus the actual additions. The energy efficiency also deteriorates because of the extra resistance generated by the material lying under the liquid metal cathode.
Studies of alumina dissolution have established that sludge formation is minimised when the alumina is added as slowly as possible thus suggesting continuous input will enable better cell performance. Accordingly, U.S. Pat. No. 5,476,574 assigned to Comalco Aluminium Ltd discloses a continuous feeder for adding alumina to the electrolytic cell. The apparatus is based on controlling the cross-sectional area of an orifice through which the powder flows by means of a connected pneumatic linear positioner. The main disadvantage of this feeder is that it is insensitive to changes in the flow and physical properties of the alumina. There is no feedback information provided from the system that can either identify or self-correct changes such as in the properties of the alumina which alter the mass flow rate relationship and therefore reduce the potential benefits of continuous feeding.
All previous feeding systems have been based on volumetric measurements and these have limited accuracies because densities of particular materials can typically vary by 10%. While application of volumetric measurements to the continuous feeding of alumina has resulted in the potential for significant improvements in feeding accuracy, the feedback information is slow and thus the corrections not optimal. In particular its lack of sensitivity to blockages from out-of-range particulate material makes it desirable to incorporate a mass flow measurement system.
New Zealand patent 234570 assigned to DSIR Industrial Development describes a slot flow meter for measuring mass flow rates of solids. This device consists of a chamber with a closed base and one or more substantially vertical slots in its sides. Particulate solids are passed through the chamber in such a way that only part of the overall slot length is occupied by the flowable material (hence the term “open slot”). In this “open slot” arrangement, particulate solids or more generally flowable material introduced into the chamber will tend to flow out through the slots at a rate proportional to the height of solids in the slots. The mass feed rate indicated by a given height is dependent on particulate properties including bulk density. It is usually impractical to measure the height of flowing solids at the slots and therefore a mass sensing device is used to determine the mass of solids in the chamber. Theoretical relationships between the mass of solids in the chamber and the solid mass flow rate have been described, particularly in the transient filling and emptying modes.
One significant disadvantage of using this “open slot” method in some environments is that a very complex mass determination means is required to determine the mass of material in the flow meter. In particular, in the application of the method to the process of electrolytic reduction, the components of the mass measurement means must be able to withstand high temperatures, electromagnetic interference and radio frequency disturbances.
According to NZ 234570 and other publications on the slot flow meter, there is an approximately straight line relationship between mass of material in the chamber of the slot flow meter, and the height of material in the chamber. However, this relationship is only approximate, and outside factors can result in significant discrepancies between the flow rate calculated by the slot flow meter and the actual flow rate. For example, at low flow rates, using currently known slot flow meters, the relationship is not directly proportional, and therefore there is some discrepancy between the flow rate calculated by the slot flow method and the actual flow rate.
Accordingly, it is desirable to have a second, reliable source of information to calibrate the slot flow meter, or alternatively, a different technique altogether which is not subject to the inaccuracies of the slot flow meter described above.
It is not a simple matter in environments such as the environment in which the alumina reduction process is carried out to set up a second system for accurately recording the flow of material. There is often a very limited space between the storage vessel and the melting cell in which to place the equipment. In addition, any equipment installed must be able to withstand high temperatures, electromagnetic interference and radio frequency disturbances.
If a different technique is desired altogether this system would also need to include components which are able to withstand the above conditions. Optical strain gauge transducers can be used to measure the mass of material in the slot meter system under these conditions, however they are complex and expensive, costing the region of $7,000 to $10,000. An alternative mass measurement means costing less money would be desirable in a new system.
The known slot flow meter system requires no prior knowledge of the material being dispensed. However, if the flow properties of the material change significantly, then the expression between mass and solid mass flow rate may also change. Such changes are known to frequently occur in alumina through segregation, particle size distribution variations, and other parameters. This results in calibration limitations in the system which limit accuracy at low flow rates. Accordingly an object of the present invention is to provide a method for calibrating the flow meter, or a different method for determining mass flow rate.
It is an object of this invention to provide an improved method and apparatus for measuring and controlling the discharge of flowable material, or at least to provide the public with a useful choice.
According to the present invention there is provided a method of determining an inlet flow rate (Finlet) of a flowable material including:                (a) passing an inlet stream of flowable material through a chamber having an outlet aperture to one end thereof;        (b) measuring a first rate of change of quantity of material in the chamber when the material is entering at said inlet flow rate;        (c) measuring a second rate of change of quantity of material in the chamber when no material is entering the chamber; and        (d) calculating the inlet flow rate Finlet from said first and second rates;        wherein steps (c) and (d) are conducted whilst the whole of the outlet aperture in the chamber is occupied by the flowable material.        
In contrast to the “open slot” arrangement, a “closed aperture” or “closed slot” flow pattern is produced when the whole of the aperture or slot is occupied by the flow of material. In other words, the level of flowable material in the chamber must be above the uppermost point of the or each outlet (drain) aperture in the chamber when conducting steps (b) and (c). A closed aperture or closed slot flow rate is approximately constant so that the rate at which solids flow or drain out of an opening can be estimated from the slope of the mass-time drainage curve.
It has been found by the applicant of the present invention that a relatively simple “two-point mass measurement” technique can be used to measure an inlet flow rate of flowable material, and that this simple technique can be utilised in a calibration system in order to provide a simple method for calibrating a continuous feeding system. In a preferred embodiment of the invention the method of determining an inlet flow rate of flowable material utilises simple and inexpensive components, which can withstand the high temperatures, electromagnetic interference and radio frequency disturbances to which the components will be exposed in preferred applications of this technique, such as in the process of electrolytic reduction of alumina.
In contrast to known slot-flow meters, this preferred flow meter of the present invention includes:                a chamber through which the flowable material can pass, the chamber including an outlet aperture at a lower end thereof and a wall defining an enclosed region above said outlet aperture,        wherein the dimensions of the wall are such that flow rates can be measured whilst the whole of the outlet aperture in the chamber is occupied by flowable material.        
The slot flow meter of the present invention may be either a modified version of the slot flow meter of NZ 234570, or may be a flow meter having no vertical elongate slot feature.
In contrast to known slot flow meters, both types of flow meter of the present invention include a wall defining an enclosed region above the outlet aperture having dimensions which are great enough to enable the flow rate to measured whilst the whole of the outlet aperture in the chamber is occupied by flowable material (ie. a “closed region” or a “closed aperture” region). Such flow meters can therefore be operated in such a way that the flowable material passing into the meter will flow under “closed slot” meter principals.
It is to be noted that the “closed aperture” flow meter of the present invention may include an open slot region for operating the meter using known open-slot principles. However, it has hitherto been unknown to provide such meters with a “closed aperture” or “closed slot” region and to calibrate the meter by reference to a closed aperture calibration sequence. Specifically, the closed slot meter theory can be utilised to calibrate the flow meter, and thereafter the actual flow rate measured by the open slot flow rate determination method can be compared to the flow rate expected at the given flow setting.
According to an alternative embodiment of the invention, the flow meter does not have an “open slot” feature. With reference to this particularly preferred embodiment of the invention, it has been found by the applicant that by combining this closed slot flow meter with simple measurement means it is possible to obtain a flow meter for calculating the solids mass flow rate of a flowable material (on an intermittent basis) which is relatively very inexpensive to install and is relatively robust in the harsh operating conditions that may be experienced in the electrolytic reduction process. Therefore a number of disadvantages associated with open slot meters can be avoided.
One principal disadvantage in the application of the known slot-flow meter to aluminium smelting applications which is avoided concerns the need in such applications to measure a wide range of feed rates. For instance, for a 170 kA cell having 4 feeders (each provided with a flow meter) under normal operating conditions each feeder supplies alumina at a rate of 5-9 g/s. However, in the event of an anode effect (where it is necessary to feed alumina into the cell as quickly as possible) the flow rate required is 35 g/s or more. The complete range of 0-40 g/s is difficult to measure accurately at both low and high flow rates in the known slot flow meter. As explained above, at low flow rates the slot flow meter is not extremely accurate.
The flow meter developed by the present application also has the advantage over the known slot-flow meter in that the outlet aperture can be sized to permit passage of oversized material. In contrast, known slot flow meters require a relatively narrow slot region.
In addition, since the flow meter of the present invention does not require a relatively long open slot region, the height of the unit can be reduced. The height of the components used in a flow meter in some environments can be critical in determining whether the flow meter is suitable for a particular application. Accordingly this is a significant advantage of the flow meter of one preferred embodiment of the invention over known slot flow meters.
For instance, in the case of the aluminium smelting industry, a number of flow meters are used at a number of feeding points to feed alumina into one cell. Each feeder (and therefore each flow meter) used must have the capacity to feed larger than average quantities of alumina into the cell in the event that one or more of the other feeders fails. Hence, for an open slot meter, twice the normal operating level of slot capacity is required. This corresponds to an open slot meter with an increased slot height, if the same mass versus mass flow rate resolution is required. Alternatively, if there is not enough space for the increased chamber height required, and a wider slot is used, resolution is compromised.
The chamber of the flow meter of the present invention may include one or more outlet apertures. In the case of a plurality of outlet apertures, the base of the chamber is shaped so as to facilitate even distribution to each outlet aperture.
Preferably, the chamber of the flow meter also includes outflow openings above the enclosed region of the chamber. The outflow openings enable overflow levels of flowable material to pass through the flow meter.
In the case where the flow meter of the present invention includes an elongate slot, the elongate slot may constitute the outlet aperture, or may be present in addition to the outlet aperture.
In the case where the elongate slot is present in addition to the outlet aperture, the outlet aperture is preferably sized to permit passage of oversized material that cannot exit the chamber of the flow meter through the elongate slot. It is also preferred that the outlet aperture be spaced apart from the elongate slot. Whilst it is not necessary for the outlet aperture and the elongate slot to be vertically aligned, in a preferred embodiment the elongate slot is spaced vertically above the outlet aperture.
According to one preferred embodiment of the invention, the chamber of the flow meter does not include an elongate slot of the type that enables the flow of material through the meter to be calculated using open slot meter calculations. According to this preferred embodiment of the invention, the chamber through which the flowable material can pass has an outlet aperture at a lower end thereof of a cross section that enables flowable material to drain from the chamber at a rate less than the minimum flow rate to be measured.
As will be explained in further detail below, the flow meter having a chamber of this configuration enables a simple two mass point calculation to be utilised to determine the flow rate of flowable material passing through the chamber.
Preferably, the flow meter includes measurement means for measuring the time taken for the mass of flowable material in the meter to pass from a first mass to a second mass. Any known means may be used to detect when the mass has passed from a first mass to a second mass.
This can be detected by recording any measurable quantity that varies proportionally to the mass of material. For example:                actual mass could be measured,        the height of material in the flow meter could be measured and converted into a mass measurement,        the pressure transducer output voltage in a pressure bellows could be measured and corresponding mass calculated;        the capacitance in a parallel plate capacitor fitted to the outlet slot of the weighing chamber could be measured and a mass calculated therefrom; or        the chamber walls could be configured to constitute a parallel plate capacitor and a mass calculated from the capacitance measured therefrom.        
Preferably, the measurement means includes a displacement means enabling the chamber to move between a first position at a first mass of flowable materials present in the chamber and a second position when a second mass of material is present in the chamber, and timing means (including, for example, an electrical circuit) by means of which the time taken for the chamber to move between said first and second positions is measured. The displacement means may be of any suitable configurations and may for example include a beam such as a carbon fibre beam or a biasing means such as spring. As explained above, it is preferred that the measurement means detects movement between two discrete positions corresponding to the first and second masses only.
According to the present invention there is provided a method for calibrating the rate at which flowable material is discharged from a storage vessel through a flow control means, said flow control means having a plurality of settings controlling the rate of flow of flowable material discharged from the storage vessel over a flow rate range between minimum and maximum flow rates corresponding to minimum and maximum flow rate settings, the method including:                (a) calculating the flow rate for a first flow rate setting of the flow control means;        (b) calculating the flow rate for a second flow rate setting of the flow control means; and        (c) calculating a flow rate versus flow control means setting expression.        
Preferably the flow rates for the first and second flow rate settings are determined by the method described in general above.
Preferably the flow control means includes a flow control valve.
The flow control valve may be of any suitable configuration. Without wishing to limit the scope of the invention, according to a preferred embodiment of the invention the flow control valve may comprise a plate having a variable shaped orifice. By changing the position or the setting of the valve relative to an outflow point of the storage vessel, the size of the opening and hence the solids mass flow rate will be caused to change.
Preferably, the method for calibrating the rate at which flowable material is discharged from the storage vessel through the flow control valve involves calculating the flow rate at a flow rate setting which corresponds to the maximum flow rate and the minimum flow rate.
Preferably, these flow rates are determined using the methods described in general above.
According to the present invention there is also provided a method of monitoring a continuous feeding system for flowable materials which flow through a flow control means having a plurality of settings, said method comprising:                (a) calibrating the rate at which flowable material is discharged to the flow control means to obtain a flow rate versus flow control means setting expression;        (b) setting the flow control means at the setting required to obtain a required flow rate as calculated by the flow rate versus flow control means setting expression; and        (c) re-calibrating the rate at which flowable material is discharged through the flow control means to obtain a re-calibrated flow rate versus flow control means setting expression.        
Preferably the re-calibration step is conducted when a precondition is met. Depending on the particular flow rate determining method in use in the system, the precondition may be one of a number of events. For example, the precondition may be that a pre-set time period has elapsed since the last calibration or re-calibration was conducted. Alternatively, the precondition may be based on a certain flow rate measurement reading taken by a second flow rate determining method (eg. the slot flow meter method). Another alternative is when the measuring signal changes outside pre-determined limits thus signifying a change in property of the material that may influence its mass flow rate.
According to one embodiment of the invention, the precondition is one of the following:                (i) that the flow rate required has changed and the previous flow rate was the maximum flow rate; and        (ii) that the flow rate required has changed, the new flow rate required is not the maximum flow rate, the setting of the flow control means is changed to correspond to the new flow rate required, the flow rate at the new flow control means setting is calculated, and the new flow rate calculated is not within a tolerance range of the flow rate expected at the new flow control means setting.        
According to an alternative embodiment of the invention, the precondition may be that the discharge flow rate at a given flow control means setting measured by a second flow rate determining method is not within a tolerance range of the flow rate expected at the given flow control means setting.